No problem at all. I had this happen once; the real mystery was how it got in (the carboy wasn't in a box or anything, so it would have to have leapt from the wall or fallen from the ceiling - I don't think mice can scale vertical glass).

then again, you might want to be adventerous, take a walk on the wild side. just leave that poor mousie in place and ferment away. sort of a cock ale variant. then you could really make use of Jeff's naming suggestions.